Photodetector imagers, in general, comprise a plurality of photodetectors arranged in either a linear or area array. Along the linear array of the photodetectors or along each column of photodetectors in an area array is a transfer device for Carrying the charges collected in the photodetectors to read-out circuitry. One transfer means commonly used is a Charge Coupled Device (CCD). The photodetectors commonly used are either a photogate detector or a photodiode detector.
FIG. 1 to which it is now made reference schematically represents a conventional image sensor such as disclosed in commonly-assigned U.S. Pat. No. 5,051,797 issued Sep. 24, 1991 to H. J. Erhardt, and comprised of a plurality of image cells 1. The image sensor comprises a substrate (semiconductor body), typically of a semiconductor material, such as a p-type single crystalline silicon on which photodetectors are formed by at least one implanted region of a conductivity type opposite to the one of the substrate and an implanted region of the same conductivity type as the substrate. Such a substrate, along with its implanted regions, defines for each image cell 1 a photosensitive region 10 for generating electric charges. Adjacent to the photosensitive region 10 is an accumulation region 11 for collecting the electric charges outside the photosensitive region. Typically, the accumulation region is formed by a conductive gate of a conductive material, such as metal or conductive polycrystalline silicon.
Electrode means are provided over the accumulation region for applying a constant potential to the accumulation region so that the electric charges generated by the photosensitive region 10 are continuously gathered in the accumulation region 11. Such electrode means are of the same type as those shown in FIG. 6B which will be used for the description of the present invention. The image cell also comprises CCD shift registers for transporting the electric charges to an output device (not shown). Typically such a shift register is comprised of two gates 13 and 14. As an example, such a shift register is comprised of a n-type conductivity channel region in the substrate. The channel region extends along the surface spaced from and parallel to the line of the accumulation regions 11. Over the channel region are a plurality of conductive gates 13 and 14. The gates may be made of a metal or conductive polycrystalline silicon. The gates are connected to bus lines (not shown) and to a potential source (not shown) for selectively applying a potential to the gates 13, 14 to operate the shift registers.
Between each accumulation region 11 and each shift register is a transfer region 12 to permit the transfer of the electric charges from the accumulation region 11 to the shift registers. Electrode means are provided over the transfer region to selectively lower the potential of the transfer region under the potential level of the accumulation region so that the transfer of the charges can be performed. Such electrode means are of the same type as those T shown in FIG. 6B.
In operation of the image sensor, a potential is applied to the electrode of the accumulation regions 11 so as to deplete them. This also induces a potential well in the photosensitive region 10, which well in each accumulation region which is deeper than the potential well 23 of the photosensitive region 10. Thus, during the integration period during which time the image sensor is subjected to scene illumination, as charge carriers are generated in each photosensitive region 10, the charge carriers diffuse or drift into its contiguous accumulation region.
As it appears in FIG. 2A to which it is now made reference, and which shows a cross-section view of the image sensor of FIG. 1, isolation regions 21 are provided between the image cells 20 to isolate the image cells from each other. Generally, the isolation regions are comprised of thick field oxide grown between two adjacent cells. Typically, such field oxide isolation regions are provided on a length which is sufficient to isolate the photosensitive regions from each other and the accumulation regions and transfer regions from each other.
Such isolation regions pose several problems to this kind of image cell structure, primarily due to the fact that after a channel stop region is patterned, the subsequent growth of the field oxide causes a lateral extension of the isolation region to occur, forming a "bird's beak" structure as it is commonly known in the prior art. This encroachment can have a typical length of 0.5 .mu.m to 1.0 .mu.m into the active region depending on the process steps involved. The channel stop is a region of near zero electric field and carriers travel through this region by diffusion. This results in a higher diffusion crosstalk between adjacent photodiodes which degrades the Modulation Transfer Function (MTF) and reduces the collection efficiency of the photodiode. This effect also tends to limit the smallest cell size achievable. These concerns are of utmost importance in the design and manufacture of high resolution image sensors.
A potential solution has been proposed in the article entitled "A 3456 Element Quadrilinear CCD with Depletion-Isolated Sensor Structure" by Declerck et al, IEDM Technical digest 1983, pp. 505-508. The sensor disclosed in this article comprises a plurality of image cells which are only separated from each other by the depletion region of the p-n junctions. No field doping, thick oxide or poly-gate isolation is used. However, as acknowledged by the authors of the article, depletion isolation sensors have been shown to have the disadvantage of the detector varying in width with respect to the signal charge level. The width variation in these sensors is due to the change in potential profiles caused by the collection of electrons. This problem consequently degrades the sensor MTF as well. Furthermore, a photosite with a depletion region isolation without any accumulation region, will exhibit a reduced charge collection efficiency due to the potential well collapsing during integration.